"WASHINGTON — Scientists think they have answered a whale of a mystery: How the ocean creatures got so huge so quickly.

A few million years ago, the largest whales, averaged maybe 15 feet long. That's big, but you could still hold a fossil skull in two hands.

Then seemingly overnight, one type of whale — the toothless baleens — became huge. Modern blue whales get as big as 100 feet, the largest creatures ever on Earth. Its skull is now bigger than a minivan and could probably fit more than five people inside, researchers said.

"We really are living in the time of giants," said study co-author Nicholas Pyenson of the Smithsonian Natural History Museum. "Why is that?"

And it happened "in the blink of an evolutionary eye," which makes it harder to figure out what happened, said Graham Slater at the University of Chicago, lead author of the study in Tuesday's Proceedings of the Royal Society B."